The present invention relates to plasma arc torches, and more particularly to a method of forming an electrode for supporting an electric arc in a plasma arc torch.
Plasma arc torches are commonly used for the working of metal, including cutting, welding, surface treatment, melting, and annealing. Such torches include an electrode which supports an arc which extends from the electrode to a work piece in the transferred arc mode of operation. It is also conventional to surround the arc with a swirling vortex flow of gas, and in some torch designs it is conventional to also envelop the gas and arc in a swirling jet of water.
The electrode used in conventional torches of the described type typically comprises an elongate tubular member composed of a material of high thermal conductivity, such as copper or a copper alloy. The forward or discharge end of the tubular electrode includes a bottom end wall having an emissive element imbedded therein, which supports the arc. The emissive element is composed of a material which has a relatively low work function, which is defined in the art as the potential step, measured in electron volts (ev), which permits thermionic emission from the surface of a metal at a given temperature. In view of this low work function, the element is thus capable of readily emitting electrons when an electrical potential is applied thereto. Commonly used materials include hafnium, zirconium, tungsten, and alloys thereof. The emissive element is typically surrounded by a relatively non-emissive separator, which acts to prevent the arc from migrating from the emissive element to the copper holder. A nozzle surrounds the discharge end of the electrode and provides a pathway for directing the arc towards the work piece.
More specifically, the emissive insert erodes during operation of the torch, such that a cavity or hole is defined between the emissive insert and the metallic holder. When the cavity becomes large enough, the arc xe2x80x9cjumpsxe2x80x9d or transfers from the emissive insert to the holder, which typically destroys the electrode. To prevent or at least impede the arc from the emissive insert to the holder, which typically destroys the electrode. To prevent or at least impede the arc from jumping to the metallic holder, some electrodes includes a relatively non-emissive separator that is disposed between the emissive insert and the metallic holder. Separator are disclosed in U.S. Pat. No. 5,023,425, which is assigned to the assignee of the present invention and incorporated herein by reference.
The assignee of the present invention has previously developed a method for making an electrode which significantly improved service life, as described in U.S. Pat. No. 5,097,111, the entire disclosure of which is incorporated herein by reference. In particular, the ""111 patent discloses a method for making an electrode which includes the step of forming an opening in the front face of a cylindrical holder or blank of copper or copper alloy and inserting a relatively non-emissive separator, which is preferably formed of silver and sized to fit substantially with the opening. Next, the non-emissive separator is axially drilled to form a cavity having a solid rear wall in one embodiment at the back of the cavity, and a cylindrical emissive element is pressed into the cavity. To complete fabrication of the electrode, the front face of the assembly is machined to provide a smooth outer surface, which includes a circular outer end face of the emissive element, a surrounding annular ring of the non-emissive separator, and an outer ring of the copper holder.
While the method of forming an electrode described by the ""111 patent provides substantial advances in the art, further improvements are desired. In particular, it has been shown that heating the electrode after the emissive element has been pressed into the separator improves the life of the electrode by forming a diffusion bond between the emissive element and the separator. However, the post-assembly heating step described above oftentimes causes the emissive element to xe2x80x9cpopxe2x80x9d or migrate out of the cavity during the heating step. This is particularly true for emissive elements that are formed out of a combination of metal powders, which typically have a density of 90-95% of theoretical. In this regard, around 5-10% of the emissive element is composed of air voids between the powdered materials. These voids expand during the heating step, which causes the emissive element to move relative to the separator.
In addition, air can be trapped between the emissive element and the separator as the emissive element is inserted in the separator, which can also expand to move the emissive element relative to the separator during the heating step. This creates a gap between the emissive element and the solid rear wall of the cavity in the separator, which decreases the heat transfer capability of the electrode. Disadvantageously, a larger percentage of the emissive element is subsequently removed during the machining step, which wastes material.
It is also desirable to limit the exposure of the emissive element to the atmosphere during the assembly of the electrode. In particular, gases from the atmosphere, such as nitrogen, can pass between the emissive element and separator during the post-assembly heating step if the emissive element is exposed to the atmosphere, which can weaken the bond or interface therebetween. Accordingly, it is desirable to form an electrode for a plasma arc torch that restricts movement of the emissive element during assembly of the electrode. It is also desirable to form an electrode for a plasma arc torch wherein the emissive element is not exposed to the atmosphere during the post-assembly heating step so that an improved bond can be formed therebetween.
The present invention was developed to improve upon conventional methods of making electrodes and those methods disclosed in the ""111 patent. It has been discovered that the difficulties of the methods described above, namely movement of the emissive element in the separator during the post-assembly heating step as well as exposing the emissive element to the atmosphere during the heating step, can be overcome by positioning the emissive element in a cavity having a solid rear wall defined by the separator, inverting the assembly, and inserting the assembly into an opening or bore defined by the metallic holder such that the emissive element is fully surrounded by the separator and the metallic holder. Thus, during the post-assembly heating step the emissive element is prevented from moving relative to the separator. In addition, the emissive element is sealed from the atmosphere after the assembly is inserted in the opening of the holder, such that gases from the atmosphere cannot enter between the emissive element and the separator during the post-assembly heating step.
More particularly, in accordance with one preferred embodiment of the present invention, a method of forming an electrode for use in a plasma arc torch comprises at least partially inserting an emissive element into a separator having an open end and a closed end. The separator and emissive element are then at least partially inserted into an opening or bore having an open end and a closed end defined by a metallic blank such that the emissive element is positioned between the closed end of the metallic blank bore and the closed end of the separator cavity. To finish the electrode, at least part of the closed end of the separator is removed so as to expose the emissive element adjacent the open end of the metallic blank.
In one embodiment, the method further comprises heating the metallic blank, separator, and emissive element to a specific temperature for a predetermined period of time. The heating step acts to cause diffusion bonding between the metallic blank, separator, and emissive element. For example, heating the electrode to a temperature in the range of around 720-800xc2x0 C., and more particularly around 750xc2x0 C., can increase the life span of the electrode by a factor of two or three. In one embodiment, the heating step includes forming thermal conducting paths, preferably formed of silver, between the emissive element and the separator. Advantageously, the emissive element is sealed and surrounded by the separator and holder during the formation of the electrode. As such, the emissive element is prevented from moving during the post-assembly heating step, and a strong bond is formed between the emissive element and the separator.
The emissive element comprises a metallic material having a relatively low work function, such as hafnium, zirconium, or tungsten. The metallic material may also include powdered mixtures and alloys thereof, which may include elements such as silver, gold, copper, and aluminum. The relatively non-emissive separator is positioned about the emissive element such that the separator is interposed between and separates the metallic holder from the emissive element at the front end of the holder, whereby the separator acts to resist detachment of the electric arc from the emissive element and attachment of the arc to the metallic holder. The separator which surrounds the emissive element is preferably formed of a metallic material, such as silver, which is described in the ""425 patent mentioned above. This serves to increase the service life of the electrode, since the silver and any oxide which does form are very poor emitters. As a result, the arc will continue to emit from the emissive element, rather than from the metallic holder or the separator, which increases the service life of the electrode. In a preferred embodiment, the separator has a tubular shape defining a cavity or opening at one end thereof and a solid wall at the other end such that the separator and the emissive element have a close-fitting relationship. In addition, the emissive element and separator can be brazed together using a brazing material, such as silver or silver alloy.
Accordingly, the present invention provides a method of forming an electrode having improved thermal conductivity by preventing movement of the emissive element relative to the separator during the heating step of forming the electrode. In addition, the present invention provides a method of forming an electrode wherein the emissive element is sealed from the atmosphere during the heating step of forming the electrode, such that gases or other materials from the atmosphere are prevented from migrating between the emissive element and the separator, which results in a stronger bond therebetween.